In conventional high-voltage circuit breakers, the arc formed during a current breaking operation is normally extinguished using sulphur hexafluoride (SF6) as quenching gas. SF6 is known for its high dielectric strength and thermal interruption capability. Pressurized SF6 is also gaseous at the typical minimum operating temperatures of a circuit breaker, non-toxic and non-flammable. Although SF6 might decompose during extinction of the arc, a substantial fraction of the decomposed SF6 recombines, which further contributes to the suitability of SF6 as a quenching gas.
However, SF6 might have some environmental impact when released into the atmosphere, in particular due to its relatively high global warming potential (GWP) and its relatively long lifetime in the atmosphere.
The GWP is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time interval, commonly 20, 100 or 500 years. It is expressed as a factor of carbon dioxide (CO2), whose GWP is standardized to 1.
So far, the relatively high GWP of SF6 has been coped with by strict gas leakage control and by very careful gas handling. Nevertheless, there is an on-going effort in the development of alternative quenching gases.
One particularly interesting candidate for substituting SF6 as a quenching gas is CO2. CO2 is readily available, non-toxic and non-flammable. As mentioned, CO2 also has a very low GWP of 1. In the amount used for a circuit breaker, it thus has no environmental impact.
In U.S. Pat. No. 7,816,618, e.g., a circuit breaker using CO2 as an arc-extinguishing gas (i.e. quenching gas) for restraining its impact on global warming is described. Furthermore, EP-A-2284854 proposes a mixed gas mainly comprising CO2 and CH4 as an arc-extinguishing medium.
However, according to U.S. Pat. No. 7,816,618, the arc extinction capability of CO2 is inferior to that of SF6. In a circuit breaker of a conventional design, a sufficient interruption performance is thus often not achieved when CO2 is used as a quenching gas. This is particularly the case for relatively high short-current and voltage ratings.
For example, the use of CO2 in a conventional circuit breaker has been described by H. Knobloch, “The comparison of arc-extinguishing capability of sulphur hexafluoride (SF6) with alternative gases in high-voltage circuit breakers”, Gaseous Dielectric VIII, Edited by Christophorou and Olthoff, Plenum Press, New York, 1998, and by F. Baberis et al., “Prove di interruzione su interruttori commerciali in gas (MT) con l'utilizzo di miscele SF6-free”, CESI Report L17918. According to the former publication, a large reduction in interruption performance resulted from the use of CO2 instead of SF6. According to the latter publication, which is directed to medium voltage applications, a very high CO2 fill pressure of 10 bar (instead of 3.4 bar for SF6) had to be used to achieve the same performance as with SF6, thus rendering the design of the insulators and of the circuit breaker more complex. Increasing the fill-pressure of a high-voltage circuit breaker by a similar factor would require an even more complex and cost-intensive re-design of the high-voltage circuit breaker. Even if a very high fill-pressure of CO2 were provided in a high-voltage circuit breaker, this would not necessarily lead to a dielectric strength equal to the one of a comparable SF6 circuit breaker, since above a certain pressure the dielectric strength of a given gas does no longer increase.